Field of the Invention
The present invention relates to a variable nozzle unit configured to adjust a passage area for (or a flow rate of) an exhaust gas to be supplied to a turbine impeller side in a variable geometry system turbocharger, and a variable geometry system turbocharger equipped with the variable nozzle unit and configured to supercharge air to be supplied to an engine side by using energy of an exhaust gas from the engine.
Description of the Related Art
In recent years, various developments have been made with regard to a variable nozzle unit to be disposed in a turbine housing in a variable geometry system turbocharger by being sandwiched between (fastened by) the turbine housing and a bearing housing. An essential configuration of variable nozzle units disclosed in Japanese Patent Application Publications No. 2009-243431 (Patent Document 1) and No. 2009-243300 (Patent Document 2) is as follows.
A turbine housing rotatably houses a turbine impeller. The turbine housing includes a turbine scroll passage which supplies an exhaust gas to the turbine impeller. Between the turbine scroll passage and the turbine impeller, a first base ring is disposed concentrically with the turbine impeller. A second base ring is provided at a position away from the first base ring in an axial direction of the turbine impeller. The second base ring is integrated with the first base ring by use of connecting pins.
Multiple variable nozzles are provided between facing surfaces of the first base ring and the second base ring. The multiple variable nozzles are disposed at equal intervals in a circumferential direction of the turbine impeller in such a manner as to surround the turbine impeller. Each variable nozzle is provided rotatably in a forward direction or a reverse direction (in an opening direction or a closing direction) about its pivot which is parallel to a pivot of the turbine impeller. In addition, a link mechanism is disposed on an opposite surface side of the first base ring from the facing surface. The link mechanism causes the multiple variable nozzles to rotate synchronously in the forward direction or the reverse direction. When the multiple variable nozzles rotate synchronously in the forward direction (the opening direction), a passage area for (or a flow rate of) an exhaust gas to be supplied to the turbine impeller side is increased. On the other hand, the passage area is reduced when the multiple variable nozzles rotate synchronously in the reverse direction (the closing direction).
A support member is provided integrally on the opposite surface of the first base ring from the facing surface. The support member includes a cylindrical portion which houses the link mechanism. The support member further includes an outer edge portion (an outer flange) formed integrally with the cylindrical portion on one side in the aforementioned axial direction (the axial direction of the turbine impeller), and an inner edge portion (an inner flange) formed integrally with the cylindrical portion on the other side in the aforementioned axial direction. The outer edge portion protrudes radially outward, whereas the inner edge portion protrudes radially inward. The inner edge portion of the support member is integrally joined to the first base ring. The outer edge portion of the support member is sandwiched between a portion of the turbine housing on the one side in the aforementioned axial direction and a portion of the bearing housing on the other side in the aforementioned axial direction. With this sandwiching, the variable nozzle unit is disposed in the turbine housing.
While the variable geometry system turbocharger is in operation, heat from a nozzle ring flows into the inner edge portion (the inner flange) of the support member and the heat is absorbed from the outer edge portion (the outer flange) of the support member by the bearing housing. Accordingly, the temperature is relatively high in the inner edge portion of the support member, and relatively low in the outer edge portion (the outer flange) of the support member.
The conventional support member includes the cylindrical portion which houses the link mechanism in order to protect the link mechanism against the heat of the exhaust gas in the turbine scroll passage and thereby to sufficiently secure durability of the variably geometry system turbocharger. Due to the presence of the cylindrical portion, the shape of the support member tends to be complex. The complex shape of the support member makes temperature distribution in the support member complex while the variable geometry system turbocharger is in operation. For this reason, the support member is thermally deformed to a large degree during the operation. For instance, the support member is thermally deformed in such a way as to be pushed outward from the inner edge portion side. In this case, the deformation is large in the first base ring, whereby the parallelism between the facing surfaces of the first base ring and the second base ring is degraded. As a consequence, the interval between the facing surfaces of the first base ring and the second base ring is locally reduced.
In order to inhibit malfunctions such as non-smoothness of the multiple variable nozzles and to secure sufficient operational reliability of the variable nozzle unit (in other words, the variable geometry system turbocharger), a nozzle side clearance is usually set slightly larger. Thus, in the variable geometry system turbocharger in operation, a minimum interval between the facing surfaces of the first base ring and the second base ring is set greater than the width (the length in the aforementioned axial direction) of each variable nozzle. On the other hand, setting the slightly larger nozzle side clearance leads to an increase in a leak current from the nozzle side clearance, and thereby degrades turbine efficiency of the variable geometry system turbocharger. Here, the nozzle side clearance means either a gap between the facing surface of the first base ring and a side surface of the variable nozzle on the one side in the aforementioned axial direction or a gap between the facing surface of the second base ring and a side surface of the variable nozzle on the other side in the aforementioned axial direction.